docs: def fitness(self, width, height):
          """
          In guillotine algorithm case, returns the min of the fitness of all 
          free sections, for the given dimension, both normal and rotated
          (if rotation enabled.)
          """
          assert(width > 0 and height > 0)
  
          # Get best fitness section.
          section, rotated = self._select_fittest_section(width, height)
          if not section:
              return None
          
          # Return fitness of returned section, with correct dimmensions if the
          # the rectangle was rotated.
          if rotated:
              return self._section_fitness(section, height, width)
          else:
              return self._section_fitness(section, width, height)